Interpreting QM without Schrödinger's equation

[*now*]

It is point 3 in The 7 Basic Rules of Quantum Mechanics!

The Copenhagen interpretation was created in the Schrödinger picture. Born used the Schrödinger picture already in the paper

• Max Born, Zur Quantenmechanik der Stoßvorgänge, Zeitschrift für Physik 37 (1926), 863–867.

where he introduced what we call today Born's rule. Scattering in the Heisenberg picture is much more difficult to motivate, and had to wait until much later

• W. Heisenberg, Die „beobachtbaren Größen“ in der Theorie der Elementarteilchen, Zeitschrift für Physik 120 , (1943), 513–538.

when Heisenberg introduced the S-matrix.

On this, I just wonder what is meant by this, because hadn’t Born already named “Quantum Mechanics” for the discontinuous nature and written about probabilities and such, all before Schrödinger came up with a different easier approach, e.g. Born, Jordan, Z. Phys.33, 479-505, 1925; rec. 11-06-1925 ?

 

[DrChinese]

Rovelli, in his recent paper, writes:

"The mistaken idea is that the quantum state ψ represents the “actual stuff” described by quantum mechanics.

...

In RQM, it is a bookkeeping of known facts, and a tool for predicting the probability of unknown facts, on the basis of the available knowledge."

I am not sure I understand Rovelli here. Prior to "collapse", the quantum state - as described by a probability wave existing simultaneously at a variety of place in spacetime - can be manipulated at those places. How can that be termed as mere "bookkeeping" as Rovelli believes?

If we have a double slit setup, or anything where interference occurs between 2 possible paths: Obviously the mutual influence of both possible paths is responsible for the observed outcome pattern. There cannot be independence (product states) for the results.

Or: You can take the output ports of a polarizing beam splitter (PBS), and those ports obviously BOTH contain something that is "real". Because you can later recombine them, and you will return them to their original input state. In other words: what comes out of the PBS is NOT one the following:

a) A V beam from the transmitted port, and nothing from the reflected port.
b) An H beam from the reflected port, and nothing from the transmitted port.

Because those beams could not be recombined to restore the original input beam. What am I missing?

 

[Jarvis323]

I am not sure I understand Rovelli here. Prior to "collapse", the quantum state - as described by a probability wave existing simultaneously at a variety of place in spacetime - can be manipulated at those places. How can that be termed as mere "bookkeeping" as Rovelli believes?

Is Rovelli's "bookkeeping" view about the wave function different from other $\psi$-epistemic interpretations?

 

[Fra]

Rovelli's view shares parts of other epistemic views, but it's still different. The similarity is also why I share a part of Rovellis reasoning, but not his final steps.

"For relative facts, every interaction can be seen as a “Copenhagen measurement”, but only for the systems
involved. Any physical system can play the role of the “Copenhagen observer”, but only for the facts defined
with respect to itself. From this perspective, RQM isnothing else than a minimal extension of the textbook Copenhagen interpretation, based on the realisation that any physical system can play the role of the “observer” and any interaction can play the role of a “measurement”: this is not in contradiction with the permanence of interference through interactions because the “measured” values are only relative to the interacting systems themselves and do not affect other physical systems.
"
-- Rovelli, p3, https://arxiv.org/abs/2109.09170

My problem with this is that, my view is taking what Rovellis says seriously (ie that any systems qualifies as an observer) AND trying to understand the origin of actual interactions in terms of inferences (unification quest), between such arbitrary observers will force us to deform QM and the reason for this is that the real observers informationa capacity, puts a constraint to the "effective theory", and I think this is not bookkeeping, I think it has deep implications for the hierarchy of physical interactions. This makes things more complicate that what I perceive that Rovellis thinks. Rovells somehow seems to look for an interpretation that realizes a good idea, buth without breaking the QM formalism. I just do not see how that is possible.

/Fredrik